1. Field of the Invention
The present invention relates to variable speed controls for AC motors, and more particularly, to those that are driven by DC sources.
2. Description of the Related Art
In line operated AC induction motors, speed/torque/efficiency is related to line voltage and frequency (voltage/frequency). For example, a 2-pole AC motor designed to operate at a line voltage/frequency of 120 volts AC and 60 Hz rotates at 3600 R.P.M. and depending on the winding characteristics will provide a given torque. If this same motor is operated at 30 Hz. (1/2 the frequency), the motor should rotate at 1800 R.P.M. (half the speed) but at greater torque with the consequent increase in power dissipation in the form of heat generated. Thus, to maintain the same torque (and similar power dissipation) at this lower frequency, the voltage needs to be reduced to one half of the original 120 volts AC, or 60 volts AC. The V/f ratio is then maintained substantially constant at 2:1. If this ratio is not maintained, then the torque at 30 Hz. will be 200% the torque at 60 Hz. with the consequent waste of power and heat.
On the other hand, if the motor needs to be operated at twice the frequency to achieve double the rotational speed (7,200 R.P.M.), the applied voltage would have to be increased to 240 volts AC to provide the same torque, thus maintaining the same 2:1 ratio. Therefore, by varying the voltage in direct proportion to the frequency variations, the torque and power dissipation is maintained constant over an angular speed range for a given motor. However, in practice, this is t to do since the line voltage in the public network is set at a constant frequency. This accounts for the inefficient behavior of most motors at start up since they are designed keeping their final speed in mind.
With the present invention, the motor torque output and power dissipation is kept constant over an angular speed range by varying the duty cycle in symmetrical signals that, when integrated, provide substantially the same amount of energy as if the voltage (in the case of the sinusoidal line voltage) had been varied. The voltage is maintained constant but as the frequency is lowered, the duty cycle is also lowered, and vice versa. Therefore, "V/f" is substantially equivalent to "duty cycle/f".